1. Field of the Invention
The present invention relates to a zoom lens system, and particularly to a zoom lens system suitable, for example, for use as a projection optical system in an image projection apparatus (such as a liquid crystal projector for projecting an image from a liquid crystal panel onto a screen).
2. Description of the Prior Art
Various types of zoom lens systems are known. For example, Japanese Laid-open Patent Application No. S64-46717 discloses a three-unit zoom lens system that consists of, from the object side a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and a third lens unit having a positive refractive power and that performs zooming from the wide-angle end to the telephoto end by moving the second lens unit along the optical axis in such a way that the distance between the second and third lens units increases.
Some zoom lens systems are designed for use as an image-sensing optical system in an image sensing apparatus (e.g. a video camera), whereas other zoom lens systems are designed for use as a projection optical system in an image projection apparatus (e.g. a liquid crystal projector). The zoom lens system disclosed in the above-mentioned Japanese Laid-open Patent Application No. S64-46717 is designed as an image-sensing optical system, and therefore, when it is used as a projection optical system, it suffers from its short back focal length and insufficient correction of distortion.
The reason why a zoom lens system to be used as a projection optical system needs to have a relatively long back focal length is that it is necessary to secure a space for a dichroic prism or the like on the reduction side of the zoom lens system. The reason why a zoom lens system to be used as a projection optical system needs to have relatively high distortion correction performance is that, in order to produce as large a projected image as possible at as short a projection distance as possible, it is necessary to use the wide-angle range of the zoom lens system where large distortion is inevitable.